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Structures and Functions in Living Organisms

Co-ordination and response — Edexcel Linear - OL Revision Notes & Topical Questions

15 min read 18 parts Lesson 15 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Nervous and Endocrine Systems, Reflex Arc and the Eye — EDEXCEL LINEAR - OL notes: Compare nervous and hormonal control, describe synapses and reflex arcs, and explain how the structures of the eye focus on near and distant objects.FeatureNervous systemEndocrine systemMade up ofN…

  • central nervous system
  • peripheral nervous system
  • neurotransmitters
  • accommodation
  • Worked example 1
  • Worked example 2

Part 1 of 6Nervous and endocrine systems compared

FeatureNervous systemEndocrine system
Made up ofNerves (bundles of neurones), brain, spinal cordGlands
SignalElectrical impulses along neuronesHormones (chemicals) carried in the blood
SpeedVery fastSlower
Effect lastsShort timeLonger
TargetSpecific muscles or glandsOne or more target organs

Hormones are carried by the blood around the whole body and alter the activity of specific target organs.

Nervous and endocrine systems compared
The glands that produce them are collectively the endocrine system.
Central nervous system
The human nervous system has the central nervous system (CNS: brain and spinal cord) and the peripheral nervous system (the nerves branching out to the body).
Receptors in sense organs detect stimuli (light, sound, touch, temperature, chemicals) and produce electrical impulses that travel along sensory neurones to the CNS.

Part 2 of 6Synapses and the reflex arc

Reflex arc linking receptor, sensory neurone, spinal cord, motor neurone and muscle
Reflex arc linking receptor, sensory neurone, spinal cord, motor neurone and muscle

Neurones do not touch. The tiny gap between two neurones is a synapse. When an impulse reaches the end of one neurone, it triggers the release of neurotransmitters (chemical messengers) that diffuse across the gap and bind to receptors on the next neurone, starting a new impulse.

A reflex response is automatic and rapid and does not involve the conscious part of the brain as coordinator; awareness occurs after the response. This minimises damage and aids survival (pain withdrawal, blinking, coughing). The reflex arc pathway:

1
Stimulus detected by a receptor.
2
Impulse along a sensory neurone to the CNS.
3
Synapse to a relay neurone in the spinal cord.
4
Synapse to a motor neurone.
5
Impulse along the motor neurone to the effector (a muscle or gland), which responds.

Part 3 of 6The structure of the eye

Anatomy of the human eye
Anatomy of the human eye

The eye is a highly specialised sense organ containing light receptor cells.

PartFunction
ConjunctivaClear membrane covering the white of the eye and inside of the eyelids; lubricates and protects from irritants
Cornea / lensRefract and focus light onto the retina
Suspensory ligamentsConnect the ciliary muscle to the lens
RetinaContains receptor cells (rods and cones) sensitive to light
FoveaRegion of the retina with densely packed cones: detailed colour vision
Optic nerveCarries impulses between the eye and the brain

Part 4 of 6Accommodation: focusing near and distant objects

Accommodation for a near object: ciliary muscles, ligaments and lens
Accommodation for a near object: ciliary muscles, ligaments and lens

Fine focusing by the lens is called accommodation. The lens is elastic and its shape changes when the suspensory ligaments become tight or loose, controlled by the contraction or relaxation of the ciliary muscles.

Near objectsDistant objects
Ciliary musclesContractRelax
Suspensory ligamentsLoose / slackTight / taut
Lens shapeFatter (more curved)Thinner (less curved)

Part 5 of 6Comparing speed, duration and examples of responses

The nervous system gives fast, short-lived responses such as pulling a hand away from a hot object, while the endocrine system gives slower, longer-lasting responses such as growth, puberty or the control of blood glucose.

Comparing speed, duration and examples of responses
Some responses use both: in a frightening situation the nervous system produces an immediate reaction and adrenaline from the adrenal glands prepares the body for sustained action.
When you answer a comparison question, use comparative words such as faster, slower, shorter and longer, and give an example of each.

The effectors in the nervous system are muscles (which contract) and glands (which secrete). Remember the sequence for a coordinated response: stimulus, receptor, coordinator (CNS), effector, response.

Part 6 of 6Worked examples

  1. Give two differences between nervous and hormonal control. [2]2 marks

  2. Describe how an impulse crosses a synapse. [3]3 marks

  3. Explain why reflexes protect the body. [2]2 marks

  4. Describe how the eye focuses on a near object. [3]3 marks

  5. State the function of the fovea. [1]1 mark

Key terms

TermMeaning
SynapseThe gap between two neurones.
NeurotransmitterA chemical released at a synapse that carries a signal across the gap.
Reflex arcThe pathway of a rapid automatic response from receptor to effector.
AccommodationChange in lens shape to focus on near or distant objects.
RetinaLight-sensitive layer at the back of the eye.
HormoneA chemical made by a gland and carried in the blood to target organs.

Avoid these mistakes

1
Saying the neurones touch at the synapse — there is a gap.
2
Mixing up ciliary muscle and suspensory ligament changes for near and distant objects.
3
Saying reflexes involve conscious decisions by the brain.
4
Saying hormones travel by nerves rather than the blood.

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